Bonded molecular sieve materials of the zeolite family are well known to those skilled in the art. These natural and synthetic materials, having different particle sizes and pore sizes, are hydrated metal alumino silicates.
It has been known for many years that molecular sieves have excellent adsorptive properties. Thus, molecular sieve materials have been used in desiccant filters. These desiccant filters have often been comprised of a metallic cartridge or vessel filled with loose, activated particles of sorbent, such as molecular sieve material. However, as is disclosed in U.S. Pat. No. 3,235,089 of Burroughs, the use of such loose sorbent particles has the following disadvantages: "(1) a tendency to develop channels, (2) attrition between individual particles which causes powdering and packing with a resultant change in the penetrability of the unit and also a lessening of desiccant properties, and (3) formation of strata in the bed due to classification which occurs during use . . ." (see column 1, lines 21-30)
Because of the problems with the use of loose sorbent particles in desiccant filters, attempts have been made to produce monolithic filter bodies of molecular sieve material. However, the prior art discloses that it is very difficult to make a filter body high in molecular sieve content which will effectively utilize the adsorptive properties of these materials.
In 1961, in U.S. Pat. No. 2,973,327, Mitchell et al. disclosed that it is difficult to agglomerate molecular sieve because ". . . the alkalinity of the sieve . . . tends to destroy or weaken many common types of binders . . . . In the presence of water, the molecular sieves dilate and thus resist compaction forces . . . (see column 1, lines 57-63)"
In 1962 it was disclosed that filter bodies which contained a substantial amount of molecular sieve material are ineffective. In his U.S. Pat. No. 3,025,233, Figert disclosed (at 1 lines 61-66 of column 3) that ". . . crystalline zeolite molecular sieves have . . . limited filtering ability, when used alone, to provide full fluid flow with a negligible pressure drop and provide the retention of finely divided particulate matter . . . ." Figert's solution to this problem was to provide a porous barrier which contained from 7 to 31 percent of a crystalline zeolite molecular sieve, from 39 to 93 percent of activated alumina, and an inert binder which could be organic or inorganic.
By no later than 1966 it was recognized that prior art binding techniques were ineffective in making shaped bodies consisting of molecular sieve. In their U.S. Pat. No. 3,262,890, Mitchell et al. taught (at lines 34-42 of Column 1) that "The prior art has employed shaped molecular sieve bodies consisting of hydrated alumina particles and 5 to 10 percent zeolite 4A particles with cellulose acetate or calcium aluminate. These bodies or blocks are shaped to fit in the sealed refrigerant cartridges. However, when the blocks are sectioned, it is found that the molecular sieve particles are quite easily dislodged from the mass." The patentees also disclosed (at lines 47-56 of column 1) that "Attempts to bind 14.times.30 mesh zeolite type 4A particles with calcium aluminate cement, or calcium aluminate and sodium silicate have proved unsuccessful. It appears that the alkaline nature of the molecular sieve particles adversely affects the binders, and that conventional binding techniques are not suitable for preparing shaped bodies containing uniformly dispersed molecular sieves." The solution to this problem presented in U.S. Pat. No. 3,262,890 was to provide a green body containing clay mineral binder and sodium silicate and to thereafter fire this body at a temperature of from about 662 to about 1,202 degrees Fahrenheit.
By no later than 1969, another problem with the use of molecular sieve material was recognized. In his U.S. Pat. No. 3,446,645, Wilfred Drost disclosed (at lines 45-49 of column 1) that ". . . under some conditions, halogenated derivatives of ethane and methane employed as refrigerant gases are decomposed to some extent by a number of prior known molecular sieve agglomerates used as desiccants in the system."
This decomposition problem was again referred to in 1970. In their U.S. Pat. No. 3,536,521, Robert W. McKinney et al. disclosed (at lines 35-40 of column 1) that ". . . the Freons . . . used in refrigerant systems react at active sites on a molecular sieve surface with subsequent decomposition into halogen acids which react with the basic structure of the molecular sieve."
A self-supporting structure containing molecular sieve and thermoplastic material was disclosed in U.S. Pat. No. 4,665,050 of Degen et al. In the process of this patent, a sorbent (such as molecular sieve) with a particle size in the range of from 1 to 10 millimeters was preheated, and the preheated particles were then mixed with from about 1 to about 7 weight percent of thermoplastic binder (such as polyethylene); thereafter, the sorbent/binder mixture was heated to about the solid-liquid transition temperature of the binding material. The body formed by this process, however, has poor mechanical properties.
In 1992, in their U.S. Pat. No. 5,114,584, applicants disclosed a filter body which contained at least about 75 weight percent of molecular sieve material and from about 9 to about 20 weight percent of nylon. Although the filter body of this invention was substantially superior to prior art filter bodies, it still suffered from certain disadvantages. In the first place, nylon is not stable when heated to its melting point in an oxygen-containing environment. In the second place, nylon is not chemically resistant to many species.
It is an object of this invention to provide an integral, porous body which contains at least about 75 weight percent of molecular sieve material.
It is another object of this invention to provide a filter body with excellent filtering properties.
It is another object of this invention to provide a filter body which will have a substantially greater capacity to pick up water than the filter body of U.S. Pat. No. 5,114,584.
It is another object of this invention to provide a filter body which will have better resistance to many chemical species than the filter body of U.S. Pat. No. 5,114,584.
It is another object of this invention to provide a filter body with excellent strength properties.
It is another object of this invention to provide a filter body with excellent desiccant properties.
It is another object of this invention to provide a filter body which is entirely compatible with refrigerant fluids.
It is yet another object of this invention to provide a filter body which will retain its mechanical properties even after it has been contacted with refrigerant fluid for a substantial period of time.
It is yet another object of this invention to provide a process for preparing a filter body which does not require the use of an inert or vacuum atmosphere.
It is yet another object of this invention to provide a process for bonding a molecular sieve composition to a substrate, such as metal.
It is yet another object of this invention to provide a composite article comprised of molecular sieve material bonded to a substrate.